Overview

The Proxy design pattern is a structural pattern that provides a surrogate or placeholder for another object to control access to it. This is particularly useful for implementing lazy loading, access control, logging, monitoring, and more. By applying the Proxy pattern, you can add a new level of abstraction which can enhance functionality or control the creation and access of resources efficiently.

Key Concepts

1. Virtual Proxy: Lazily initializes an object, loading it on-demand.
2. Protection Proxy: Controls access to the original object, enforcing permissions.
3. Remote Proxy: Represents an object that resides in a different address space or network, managing its operations remotely.

Common Interview Questions

Basic Level

1. What is the Proxy design pattern and why would you use it?
2. Can you implement a simple logging proxy in C#?

Intermediate Level

3. How does the Proxy pattern differ from the Decorator pattern?

Advanced Level

4. Explain how you would use a Proxy pattern to implement lazy loading for a resource-intensive object in C#.

Detailed Answers

1. What is the Proxy design pattern and why would you use it?

Answer: The Proxy design pattern provides a substitute or placeholder for another object to control access to it. It is used to add a layer between the client and the real object for different purposes like security, managing expensive object creation, and adding other functionalities such as logging and monitoring without changing the original object's code.

Key Points:
- Provides an additional level of indirection to support distributed, controlled, or intelligent access.
- Can add functionality to the original object without changing its code.
- Useful in scenarios where instantiating an object is resource-intensive or needs access control.

Example:

public interface ISubject
{
    void Request();
}

// RealSubject class
public class RealSubject : ISubject
{
    public void Request()
    {
        Console.WriteLine("Request made to RealSubject.");
    }
}

// Proxy class
public class Proxy : ISubject
{
    private RealSubject _realSubject;

    public void Request()
    {
        if (_realSubject == null)
        {
            Console.WriteLine("Proxy: Initializing RealSubject.");
            _realSubject = new RealSubject();
        }
        Console.WriteLine("Proxy: Forwarding request to RealSubject.");
        _realSubject.Request();
    }
}

2. Can you implement a simple logging proxy in C#?

Answer: A logging proxy can intercept method calls to an object and log these calls without modifying the original class's behavior. This is useful for debugging and monitoring purposes.

Key Points:
- Implements the same interface as the target object.
- Intercepts method calls, adds logging, and then delegates the call to the real object.
- Transparent to the client which uses it as if it were the real object.

Example:

public interface IService
{
    void PerformAction();
}

public class Service : IService
{
    public void PerformAction()
    {
        Console.WriteLine("Service action performed.");
    }
}

public class LoggingProxy : IService
{
    private readonly IService _service;

    public LoggingProxy(IService service)
    {
        _service = service;
    }

    public void PerformAction()
    {
        Console.WriteLine("LoggingProxy: Before performing action.");
        _service.PerformAction();
        Console.WriteLine("LoggingProxy: After performing action.");
    }
}

3. How does the Proxy pattern differ from the Decorator pattern?

Answer: Both the Proxy and Decorator patterns use composition and delegation to perform tasks, but their intents differ. The Proxy pattern controls access to an object, adding a level of indirection for various purposes like security or lazy initialization. In contrast, the Decorator pattern is used for adding behavior or responsibilities to an object dynamically, without affecting other instances of the same class.

Key Points:
- Intent: Proxy controls access; Decorator adds responsibilities.
- Application: Proxy is used for access control, lazy loading, etc., whereas Decorator is used for enhancing functionality.
- Implementation: Both may look similar but serve different purposes.

Example: Not applicable for conceptual explanation.

4. Explain how you would use a Proxy pattern to implement lazy loading for a resource-intensive object in C#.

Answer: Lazy loading delays the initialization of an object until the point at which it is needed. This can improve performance and reduce system resource usage. The Proxy pattern can encapsulate the logic for lazy loading, initializing the real object on its first method call.

Key Points:
- Reduces memory footprint and improves application startup time.
- Encapsulates the complexity of lazy initialization.
- The proxy object acts as a stand-in for the real object until it's needed.

Example:

public interface IExpensiveObject
{
    void Process();
}

public class ExpensiveObject : IExpensiveObject
{
    public ExpensiveObject()
    {
        Console.WriteLine("ExpensiveObject created.");
    }

    public void Process()
    {
        Console.WriteLine("Processing complete.");
    }
}

public class ExpensiveObjectProxy : IExpensiveObject
{
    private ExpensiveObject _expensiveObject;

    public void Process()
    {
        if (_expensiveObject == null)
        {
            Console.WriteLine("ExpensiveObjectProxy: Initializing ExpensiveObject.");
            _expensiveObject = new ExpensiveObject();
        }
        _expensiveObject.Process();
    }
}

This implementation demonstrates how the Proxy pattern can manage the lifecycle of a resource-intensive object, initializing it only when its functionality is actually required, thereby achieving lazy loading.